The present invention relates to a developing device that develops a latent image formed on a photosensitive body (image carrier) with toner.
Japanese Patent Publication No. 5-158340A discloses an electrophotographic developing device of a contact and one component type. This developing device includes a developing roller placed rotatably while being kept in contact with an image carrier, and a supply roller placed rotatably in the same direction as the developing roller while being kept in contact with the developing roller. The developing device also includes a bias power supply, comprising resistance and a DC power supply for applying a predetermined bias voltage to the developing roller and the supply roller. The resistance is provided between the power supply and a current path branch for the developing roller and the supply roller.
It is also known a developing device of a jumping development type in which a jumping phenomenon of toner is induced by applying an AC superimposed bias voltage, in which an AC voltage is superimposed on a bias DC voltage, while the developing roller is kept in a non-contact state with respect to the image carrier. It is possible to provide individual power supplies for the developing roller and the supply roller in such a non-contact type developing device. However, in order to save the manufacturing costs, it is preferable to apply an AC superimposed bias from a single power supply to the developing roller and the supply roller as in the configuration disclosed in the above publication.
In such a case, there are provided: a first circuit originated from the power supply and returned to the power supply by way of the image carrier and the developing roller; and a second circuit originated from the power supply and returned to the power supply by way of the image carrier, the developing roller, and the supply roller. Here, a resistance value in the second circuit becomes larger than that in the first circuit by a sum of actual resistance of the supply roller and contact resistance between the supply roller and the developing roller via toner. This lessens a current flowing from the developing roller to the power supply by way of the supply roller in the second circuit in comparison with a current flowing from the developing roller to the power supply in the first circuit. It is thus difficult for negatively charged toner to move from the supply roller to the developing roller, and the toner supplying property is thereby deteriorated.